The present invention relates to improvements in the method, apparatus and materials for thermal transfer recording using a thermal transfer material and a recording medium (image receiver).
There is known the method by which said thermal transfer material is transferred imagewise to said recording material using a thermal transfer sheet including a thermal transfer layer which is rendered transferable to said recording medium (image-receiving material) as its viscosity is decreased by imagewise temperature control using a thermal recording head or the like. As examples of the thermal transfer sheet for use in the above thermal transfer recording method, there has been proposed a thermal transfer sheet comprising a heat-resistant base sheet and, as disposed on one side of said heat-resistant base sheet, a thermal transfer layer comprising an ink material comprising a binder and a colorant and having surface corrugations formed by incorporation of solid particles having a diameter in excess of the thickness of the layer, wherein the viscosity of said ink material is subject to controlled decrease by imagewise temperature control so as to be rendered transferable to a recording medium ((Reference is made, for example, to Japanese Patent Application No. 59-227155 (corresponds, in part, to U.S. Pat. No. 4,819,010)).
This type of thermal transfer sheet is such that as a recording medium is pressed against its surface and a thermal recording head is applied against the reverse side of its heat-resistant base sheet to thereby impart controlled temperature increase thereto, the ink material whose viscosity is depressed according to the amount of thermal energy so applied is caused to penetrate and be transferred to the recording medium across the surface of said solid particles and as the thermal transfer sheet is peeled off the recording medium, the non-penetrating ink material is transferred, along with the solid particles to which it is adhered, to the recording medium to yeild a mono-color reproduction image or a full-color reproduction image through superimposition of 3 or 4 primary colors, which image having a continual gradient according to the varying amount of heat applied by the thermal recording head.
However, since the image transfer by this recording technique requires a uniform contact of solid particles with the recording medium, the latter must have a smooth surface. As the recording medium, paper such as wood-free paper or coated paper or a synthetic paper (for example, YUPO, Oji Yuka Synthetic Paper Co., Ltd.) have heretofore been employed. However, paper which is fibrous is generally poor in surface texture even after calendering so that the reproduced image quality is not high. While coated paper has a smoother surface, it is hygroscopic, as a general trail of paper, so that in the thermal transfer process under highly humid conditions, the coefficient of thermal conductivity is substantially increased in the thickness direction of the paper so that the thermal energy supplied by the thermal recording head tends to esccape in the thickness direction, with the result that the thermal energy required for melting the ink material is wasted to interfere with stable thermal transfer recording. Because of its low hygroscopicity, synthetic paper is indifferent to humidity but has a surface finished coarse for improved printability. Therefore, although it is not as fibrous as genuine paper, synthetic paper is unsuited for thermal transfer recording, providing only a poor image quality. Calendered sythetic paper has an improved surface smoothness but is as much deficient in flexibility and in cushioning property in thickness direction so that as the thermal transfer sheet is pressed against it, the solid particles in the thermal transfer layer are not well brought into contact with the surface of the synthetic paper and the ingress of the solid particles into the surface layer of the synthetic paper is attenuated, thus making the particles difficult to be transferred. Thus, with synthetic paper, it is difficult to achieve improvements in image quality and in the sensitivity of thermal transfer recording.